What is the minimum amount of beverage to be used for Shabbos morning kiddush? Does it have to be alcohol?
It isn’t clear to me if you mean to ask how big the cup has to be, or how much you have to actually drink. The cup should be able to hold a reviis. For the daytime Kiddush we can use the smaller size reviis, which is 86cc, or a little bit less than a three ounce cup.
The amount that we have to drink is “kimolei lugmav”- one cheek full, which is most of a reviis. If you can’t drink that, you may give take a taste and give the rest to someone else to drink it. If you can’t even do that someone else who was yotza Kiddush with you, may drink the whole amount.
Although it is preferred to use an wine, you do not have to specifically use an alchoholic beverage, and grape juice is permitted l’chatchila.
O:CH 271-13, 14, M:B ibid 68, 69, O:CH 272-2, M:B Ibid 5.